Oil field operators demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
In wireline logging, a probe or “sonde” is lowered into the borehole after some or the entire well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole. The direct electrical connection between the surface and the sonde provides a relatively large (but not unlimited) bandwidth for conveying logging information.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous, and often more accurate, formation measurements, it is difficult to establish and maintain a direct electrical connection in an LWD environment. Consequently, alternative communication channels are typically employed for obtaining LWD logging information. Contemplated LWD communication channels include mud pulse signaling, through-wall acoustic signaling, and electromagnetic wave signaling. In each of these channels, the useful bandwidth is highly restricted relative to wireline logging.
As new tools and measurement techniques are developed the demand for higher information rates continues to increase beyond existing telemetry channel capacities.
While the disclosed embodiments are susceptible to various modifications and alternative forms, specific examples are given for illustrative purposes in the drawings and the following description. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.